This invention relates to a gasoline dispensing and vapor recovery system and method and, more particularly, to such a system and method for controlling the flow of a mixture of gasoline vapor and air from a vehicle fuel tank as it is being filled with gasoline.
A number of systems and methods have been proposed for controlling the flow of a mixture of air and hydrocarbon vapors (hereinafter referred to as "vapor/air mixture" displaced from a vehicle tank during the dispensing of gasoline into the vehicle tank at a service station, or the like, in order to reduce vapor emissions at the interface between the vehicle and the dispensing nozzle. In general, gasoline dispensing and vapor recovery systems and methods of this type include a plurality of dispenser housings with each housing being connected to an underground storage tank for gasoline. Each dispenser housing has one or more nozzles for dispensing the gasoline into a vehicle fuel tank, and passages are provided in each nozzle for collecting the vapor/air mixture from the vehicle tank. A return line is connected to the vapor/air mixture passage for delivering the collected vapor/air mixture back to the underground fuel storage tank.
Some of these systems and methods, often termed passive systems, rely solely upon vapor/air mixture pressure within the fuel tank to force the vapor/air mixture through the vapor/air mixture return line. However, due to pressure losses and partial obstructions in the vapor/air mixture recovery line (sometimes caused by fuel splash back or condensation), the vapor/air mixture pressure developed in the vehicle fuel tank was often insufficient to force the vapor/air mixture out of the vehicle tank and to the underground storage tank.
To eliminate this problem, "active" vapor recovery systems and methods have evolved that employ a vacuum pump for drawing the vapor/air mixture from the vehicle tank and through a vapor/air mixture return line. Some of these systems, such as the system disclosed in co-pending patent application Ser. No. 08/515,484, assigned to the assignee of the present invention, provide a relatively powerful, continuously-operating, vacuum pump and a valve arrangement for connecting the various vapor/air mixture return lines to the vacuum pump. Other active systems, such as a system marketed by the assignee of the present invention under the "WAYNE VAC" designation, employ a vacuum pump at each dispenser housing which is driven by the dispensing unit's conventional gasoline flow meter and which is connected to a vapor/air mixture return line.
Recent government-promulgated rules require, or will require, that onboard vapor recovery systems (ORVR) be installed on at least a portion of gasoline-operated vehicles. These systems are designed to capture and retain the gasoline vapors generated during refueling in an activated carbon canister located on the vehicle. The vapors captured in the canister will then be burned in the engine during normal driving.
Although the ORVR systems will render the above-mentioned vapor recovery systems unnecessary, the latter systems must remain in operation to service the vehicles not equipped with the ORVR systems. Therefore, when an ORVR-equipped vehicle is serviced, the vapor recovery systems will ingest some air to replace the fuel withdrawn from the storage tank. This upsets the dynamic equilibrium in the system and causes some of the gasoline in the storage tank to evaporate. The resulting gasoline vapors "grow" until dynamic equilibrium is regained and the mixture becomes saturated. This evaporation, or vapor growth, will often cause the volume of vapor in the storage tank to exceed the capacity of the system, and significant quantities of the gasoline vapor will be discharged into the atmosphere through a vent pipe associated with the storage tank. This reduces the efficiency of the gasoline dispensing system and pollutes the atmosphere.
Another major problem that is caused by a significant quantity of air being present in the vapor/air mixture recovered by the vapor recovery system and introduced into the storage tank, since, if a relatively small amount of gasoline vapor is in the mixture, the mixture may become flammable and cause flame propagation if a flame, or spark, is initiated, which could be disastrous. More particularly, if the percentage of vapor in the vapor/air mixture in the vapor recovery system drops to a certain level, flame propagation can occur. For example, it is well documented that, with respect to most gasolines dispensed at service stations, flame propagation can occur if the percentage of gasoline vapor in the vapor/air mixture is between approximately 2%-8%. (If the percentage of vapor is below approximately 2%, then the danger of flame propagation severely diminishes due to the lack of vapor in the mixture.)
Although there have been several techniques proposed, such as infra red light absorption, light refraction, and electrochemical sensing, for sensing or measuring the amount of vapor or air in a vapor/air mixture, these techniques suffer in several respects. For example, they are relatively expensive, bulky and/or delicate. Also, they can be unstable, unresponsive, and sensitive to environmental conditions. Further, some of these techniques require a relatively large amount of power and are relatively slow to recover after liquid saturation.
Therefore, what is needed is an active vapor recovery system and method which senses and responds to a predetermined percentage of vapor or air in the vapor/air mixture by shutting off the flow of the mixture from the vehicle. Also needed is a system and method of the above type which is relatively inexpensive, compact, rugged and stable. Also what is needed is a system and method of the above type which is not sensitive to environmental conditions and very responsive, yet enjoys low power consumption and recovers quickly after liquid saturation.